Automatic orthophoto printer and display including position error compensation for photo-positioning transport

ABSTRACT

An automatic orthophoto printer and display for the preparation of orthophotographs from a pair of stereo aerial photographs is described which includes circuitry for compensating for position errors of the transports holding the photographs. Position sensors provide digital position signals representing the actual position of each transport in X and Y directions to digital comparators which subtractively combine the position signals with digital stage coordinate signals representing desired position of each transport in X and Y. The resultant error signals are used to proportionately shift the rasters of video scanners scanning a portion of each photograph. A specific embodiment of a photopositioning transport and a position sensor comprising an incremental encoder is also described.

United States Patent Crawley Oct. 21, 1975 AUTOMATIC ORTHOPI-IOTOPRINTER AND DISPLAY INCLUDING POSITION ERROR COMPENSATION FORPHOTO-POSITIONING TRANSPORT Inventor: Barry G. Crawley, North Burnaby,

Canada Assignee: Hobrough Limited, Vancouver,

Canada Filed: Dec. 17, 1973 Appl. No.: 425,541

Published under the Trial Voluntary Protest Program on January 28, 1975as document no. B 425,541.

[52] US. Cl 356/2; 250/558 [51] Int. Cl. ..G01C 11/18 [58] Field ofSearch 356/2; 250/558 [56] References Cited UNITED STATES PATENTS3,564,133 2/l97l Hobrough 356/2 3,659,939 5/1972 Hobrough U 356/23,674,369 7/1972 Hobrough............................... 356/2 PrimaryExaminerRonald L. Wibert Assistant Examiner-Richard A. RosenbergerAttorney, Agent, or Firm-Christensen, OConnor,

Garrison & Havelka [57] ABSTRACT An automatic orthophoto printer anddisplay for the preparation of orthophotographs from a pair of stereoaerial photographs is described which includes circuitry forcompensating for position errors of the transports holding thephotographs. Position sensors provide digital position signalsrepresenting the actual position of each transport in X and Y directionsto digital comparators which subtractively combine the position signalswith digital stage coordinate signals representing desired position ofeach transport in X and Y. The resultant error signals are used toproportionately shift the rasters of video scanners scanning a portionof each photograph. A specific embodiment of a photo-positioningtransport and a position sensor comprising an incremental encoder isalso described.

10 Claims, 4 Drawing Figures 52 X WAUAX US. Patent 06:. 21, 1915Sheet1of3 3,914,051

AUTOMATIC ORTHOPI-IOTO PRINTER AND DISPLAY INCLUDING POSITION ERRORCOMPENSATION FOR PHOTO-POSITIONING TRANSPORT FIELD OF THE INVENTION Thisinvention generally relates to the field of automatic orthophotoprinters and displays and more particularly to an improvement whichprovides compensation of position errors encountered with mechanicalphoto-positioning systems used in such printers and displays.

BACKGROUND OF THE INVENTION An improved system which operatesautomatically to produce an orthophotograph from one or more pairs ofstereo photographs is described in U.S. Pat. Nos. 3,659,939 and3,674,369, both entitled Automatic Orthophoto Printer" by Gilbert L.Hobrough, and assigned to the assignee of the present invention. Thissystem greatly reduces the time necessary to produce an orthophotographand includes first and second photo-scanning devices which are operatedin synchronism to provide video signals for each of the two photographsmaking up a stereo pair. Homologous areas of the photographs arescanned, with the portion under consideration being termed thecorrelation zone." In order to reduce X parallax errors and otherhigherordered errors of the video signals within the correlation zone,the system includes a correlation network which operates on the videosignals to determine the amount of X parallax error, and imagetransformation circuitry and raster-shaping circuits controlled therebyfor altering the scanning patterns of the two photoscanning devices.Once X parallax for a center-point in the correlation zone has beenreduced by movement of appropriate photo positioning devices, the systemalters the scan pattern of one or both photo-scanning devices to reducethe parallax and other errors throughout the correlation zone. One ofthe resultant video signals, which represents one image of thecorrelation zone, is supplied to a cathode ray tube for imprinting onthe photographic negative. By investigating a number of correlationzones, an orthophotograph can be formed.

ln the printing of orthophotographs by such a system, it is imperativethat center-point parallax and other errors be reduced to a very lowlevel or to zero in order to obtain a desired degree of accuracy ofrepresentation and precision in detail in the resultant orthophotograph.lt is therefore very important to have, in such a system, a photopositioning device capable of rapidiy and accurately moving a photographto a position where any selected portion thereof has exactly knowncoordinates relative to the scanning device.

Such an accurate photo-positioning device is described and claimed inU.S. Pat. No. 3,687,547, by Gilbert L. Hobrough and George A. Wood,entitled Photo Positioning System," which is also assigned to theassignee of the present invention. ln such a device, the photograph iscarried by a frame assembly positioned for movement on a flat supportsurface. A photo-scanning device, such as a flying spot scanner, isaligned with an opening in the support surface and with intermediateoptics and is adapted to scan the portion of the photograph aligned withthe opening by means of a scanning raster. First and second sides of thephoto-holding frame are accurately machined as flat surfaces whichintersect at an angle of 90. A pair of toothed rack elements each carryat one end thereof a frame-abutting end member, with each such endmember having a pair of rollers maintained in engagement with anassociated one of said flat side surfaces of the frame assembly. Theracks extend from the abutting end members in a manner such that theracks extend perpendicular from the edge of the frame. These racks areeach driven by a drive gear carried on the ends of the drive shaft by apair of electric stepping motors. These stepping motors receive theinput signals for positioning of the photograph.

in an orthophoto printer system such as previously described, the inputsignals to the stepping motors may comprise desired signals foreffectively moving the optical center of the scanning rasters to aselected pair of coordinates in the photograph to define a correlationzone by the two scanning areas. As the initial portion of thecorrelation process proceeds, X and Y parallax at the center point ofthe correlation zone is generally remo ed by changing the input signalsto one or both of the photo-positioning devices associated with the leftand right stereo photographs. It can therefore be seen that theresolution of X parallax reduction that can be accomplished, using thistechnique, is directly related to the accuracy and precision of thephoto-positioning devices. Any unknown error in the positioning of thetwo photos results directly in incorrect parallax measurements andconsequently incorrect height calculations by the system.

Although devices such as described and claimed in the aforementionedU.S. Pat. No. 3,687,547 have proved quite acceptable for the resolutionrequired to make typical orthophotographs, they have proved somewhatinadequate when it is desired to use a similar system to derive andprint contour information on the orthophotograph. In such a case, a veryhigh degree of resolution of terrain height, and thus of parallax, isrequired. Because the photo-positioning devices are mechanical, theyproduce errors arising from nonlinearities in the mechanical surfaces,and additionally arising from wear and slight damage. Moreover, theirposition resolution capability is limited by the finite step incrementafforded by the stepping motors and drive trains coupling those steppingmotors to the frame assembly holding the photograph.

It is therefore an object of this invention to provide a highly accurateand precise positioning system, primarily useful in automatic orthophotodisplay and printing systems.

It is another object of this invention to provide an automaticorthophoto printer having improved error transformation capability dueto the reduction of photo-positioning errors therein.

It is yet another object of this invention to provide an apparatus forcompensating between errors resulting from the difference between adesired theoretical position for an object carried by anobject-positioning device and the actual position thereof.

It is still a further object of this invention to provide an apparatusfor accurately and precisely sensing the actual position of anobject-positioning device useful in an automatic orthophoto printer.

SUMMARY OF THE INVENTION These objects and others are achieved, briefly,by accurately and precisely sensing the actual position of a carriage inthe transport which bears the photograph, comparing that actual positionwith a desired position therefor, converting a resultant error signalinto an analog signal, and modifying the X or Y position of a raster onan apparatus for scanning the photograph in direct proportion to saidanalog signal.

BRIEF DESCRIPTION OF THE DRAWINGS The above as well as additionaladvantages and objects of this invention will be more clearly understoodwith reference to the following description, taken in conjunction withthe accompanying drawings, in which:

FIG. 1 is a combined schematic and block diagram showing an automaticorthophoto printing system;

FIG. 2 is a combined pictorial and schematic diagram showing aphoto-positioning device and one embodiment of the position sensor ofthe present invention;

FIG. 3 is a logic signal diagram for illustrating the operation of theposition sensor of FIG. 2; and,

FIG. 4 is a block diagram illustrating one embodiment of the rastershaper in the printing system of FIG. 1.

DESCRIPTION OF A PREFERRED EMBODIMENT In FIG. 1, diapositives 12A and123 corresponding to the stereo aerial photographs are mounted on thecarriages 17 and 18 on the scanner and plate transport assemblies 41 and42. The carriages l7 and 18 are adapted for movement in the X and Ydirections by the X drive motors 19 and 20 and the Y drive motors 21 and22. Light sources 23 and 24 for this particular scanning techniqueilluminate the diapositives and provide light for the scanners 2S and 26which are conventional TV pickup units, however, flying spot scannerscan also be used. A scanning printer 43 contains the cathode ray tube 59and an optical system for printing on sensitive film 43A. A computer 29,which can be any of a number available on the market, solves the basicresection-intersection equations and delivers stage coordinate signalsto the transports 41 and 42 and to printer 43, on lines 44X, 44Y, 45X,4SY and 46X, 46Y, respectively. The electronic viewer 47 enables anoperator to observe the images being scanned. A steering control 48delivers instructions to the computer during manual operations. Anelectronic correlator 49 generates X and Y parallax error signals inresponse to timing differences between corresponding elements of theleft and right video signals on output lines 50 and 51 from scanners and26.

The operation of the system illustrated in FIG. 1 is as follows. Asequential program within the computer 29 establishes a pair of modelcoordinates for examination. These model coordinates are stored in theform of highly precise digital numbers. From the stored modelcoordinates and from the X and Y parallax signals on lines 52 and 53,the computer 29 calculates stage coordinates, again in the form ofhighly precise digital numbers, and develops therefrom stage coordinatesignals. First, a set of stage coordinate signals is delivered to theprinter 43 along lines 46X and 46Y, causing the sensitive film 43A inthe printer 43 to assume a position corresponding to the selected modelcoordinates. Second, a set of stage coordinate signals for the left andright scanners is computed on the basis of an initial or arbitratryterrain height (2) evaluation for the center of the scanning area. Suchcoordinate signals are delivered on lines 44X, 44Y and 45X, 4SY toactuate the transports 41, 42, respectively, so that the diapositives12A, 12B assume a position corresponding to the desired stagecoordinates.

The correlator 49 operates on the left and right video signals, on lines50 and 51, respectively, to determine the X and Y parallax errors of thescanned area. The X parallax error signal from the correlator 49 isdelivered to the computer along line 52. On the analysis of this signal,the computer 29 can order a modification of the initial Z value in adirection that will reduce the center X parallax error to zero. Thecomputer reevaluates the stage coordinates of the scanners on the basisof the new Z value and delivers modified stage coordinate signals to thetransports 41, 42 on lines 44X, 44Y and 45X, 45Y, respectively. Themotions of the transports 41, 42 are electronically compensated for bythe simultaneous cancellation from the memory" of the correlator 49 ofthe center point Z value. In this manner, the range" of the correlatorsmemory is optimized about the center point of the scanned area.

The process of analysis of the X parallax error on line 52 and thedetermination of the new 2 value continues iteratively until thecenter-point X error (a zero-order signal) has been reduced to anacceptable level that has made optimum use of the correlators memory.

An average Y parallax signal on line 53 is also delivered to thecomputer 29 and is used during setup and orientation of the model togenerate new stage coordinate signals for the scanners in the Ydirection. After orientation, the Y parallax signal should be zero.During compilation of the model, the computer 29 responds to Y parallaxerror signals on line 53 to compensate for the effects of filmshrinkage, optical distortions, and so forth.

The printer 43 shown in FIG. 1 produces an orthophotograph on sensitivefilm 43A therein. The cathode ray tube in the printer 43 is scannedsynchronously with the scanner 25 and 26 and one of the video signalsfrom the scanners is used to modulate the light intensity of thescanning spot in the printer. In FIG. I, the video signal from a left orright scanner is delivered along line 50 or 51 through the scannerselector switch 54 and along line 55 to printer 43. Normally, the leftvideo signal is selected for printing areas towards the left of themodel, and the right video signal is selected for printing areas towardthe right of the model. For this purpose, a left-right signal fromcomputer 29 is delivered to selected switch 54 along line 55A. Thecomputer also delivers an inhibit signal on line 56A that is combined insumming circuit 57A with the video signal from switch 54. The inhibitsignal blanks the light output from cathode ray tube 59 to zero exceptduring the desired printing period.

The scan generator 56 produces deflection waveforms required forscanning the diapositives and the sensitive film. The scanning patternor raster" is normally square, but, as discussed below. the rastersignals for scanners 25 and 26 are shaped as required for registration.In FIG. 1, the deflection waveforms from scan generator 56 are deliveredalong line 58 to the printing cathode ray tube 59, and via lines 57 and59A to the raster shaper 62 for the left scanner camera, and via lines57 and 60 to the raster shaper 63 for the right scanner camera. Scanningreference signals via lines 57 and 61 are delivered to the correlator49.

The raster shapers 62 and 63 both receive AZ, or height, signals fromthe correlator 49 along lines 64 and 65. Raster shapers 62 and 63 alsoreceive signals (K1, K2, K3, K4, K and [(6, FIG. 4) from computer 29along lines 66 and 67, respectively.

The raster shapers 62 and 63 modify the square raster waveforms from thescan generator 56 delivered on lines 59A and 60, in a manner more fullydetailed hereinafter, to produce raster waveforms on lines 66A and 67Athat produce in scanners 25 and 26 rasters that are distorted by highorder transformations from their normal square shape. By this means, theleft and right stereo images are transformed in such a manner that thevideo signals on lines 50 and 5! become more similar, and the image inthe scanning printer 43 reflects the corrections for scale, parallax andother distortions arising out of non-orthogonal conditions when thepictures were taken.

To summarize the operation of the system as it has been described sofar, the orthophotograph is constructed by first establishing a pair ofmodel coordinates within the computer 29, then calculating stagecoordinates therefrom and delivering representative stage coordinatesignals via lines 46X, 46Y to the motors of the printer 43, so as tocause the film 43A to assume a position corresponding to the selectedmodel coordinates. At this point in time, the cathode ray tube 59 isblanked by the inhibit signal appearing on line 56A. Subsequently, thecomputer causes the scanners 25 and 26 to reduce the center-point Xparallax error to an acceptable level or to zero by causing thetransports 41, 42 to move in iterative steps in response to successivesets of stage coordinate signals delivered on lines 44X, 44Y, 45X, 45Y.Thereafter, higher order parallax and other errors are reduced to zeroin response to the signals from the correlator 49 and computer 29, withsimultaneous operation of the distortion of the rasters of scanners 25and 26 by the rastershapers 62 and 63.

When X parallax and other errors are eliminated, the inhibit signal isremoved from line 56A and the video output of either scanner 25 or 26,as determined by the left-right signal on line 55A, is coupled via line55 to the cathode ray tube 59. Accordingly, the corrected video signalappears as an image on the face of the scanner 59. This image willthereafter expose a portion of the film 43A. After exposure is complete,the computer 29 selects new model coordinates and delivers appropriatecenter-point stage coordinate signals via lines 46X, 46Y so that theprinter 43 assumes a new position. Thereafter, the cycle is repeated.

When the entire overlap area of the stereo photographs has been treated,the orthophotograph appears to comprise a plurality of adjacent patches,one patch for each set of model coordinates that has been selected.

As can be readily appreciated from the foregoing description, whichgenerally corresponds to that of the systems described in US. Pat. Nos.3,659,939 and 3,674,369, previously referred to, position errorsencountered with the transports 41, 42 establish a lower resolutionlimit for reduction of X and Y parallax errors. To compensate for theseerrors, the system of the present invention includes position sensors116x, 118x, associated with transports 42, 41, respectively, and coupledto carriages 1B, 17 so as to measure the actual carriage displacement inthe X direction from a reference position. Similarly, position sensorsII6Y, 118Y are provided and are associated with transports 42, 41 andare coupled to carriages 18, 17, respectively, to sense the actualcarriage displacement in the Y direction from a reference position.Position sensors 6X, ll6Y provide corresponding position signals onlines 122x, 122Y, to a comparator 124, and position sensors 118x, llBYsupply corresponding position signals on lines 123X, 123Y to acomparator 126.

As previously discussed, computer 29 has stored therein calculated stagecoordinates corresponding to the desired actual location of thecarriages l8, l7. Digital signals representing the desired X and Ycoordi-' nates are accordingly supplied on lines 120x and 120Y tocomparator 126, for transport 41, and on ines 121X and 121Y tocomparator 124, for transport 42. The digital signals provided on lines120x, 120Y, and nut, l2lY differ from the stage coordinate signalsprovided on lines 44X, 44Y, 45X, and 45Y, in that the latter are drivingsignals necessary to cause the motors 19, 20, 21 and 22 of thetransports 41, 42 to move carriages l7, 18 to the nearest motor-steppositions, whereas the former are accurate and precise digitalrepresentations of the desired positions.

Comparators 124, 126 subtractively combine the desired stage coordinatepositions with the actual positions which reflect all the mechanicalerrors of the system as well as the quantitation effects of the steppermotors and provide corresponding output signals on lines 125X, 125Y,127x, 127Y, which represent the difference therebetween. The signals onlines 125x, 125Y, 121x, 127Y, instead of being applied directly to themotors 19, 20, 21 and 22, as would be customary in servo controlsystems, are applied to the raster shapers 62, 63 in such a manner so asto produce raster waveforms on lines 66A and 67A that produce inscanners 25 and 26 rasters that are distorted from that normallyproduced by a DC shift in either the X or the Y directions, or both, tocompensate for the position errors encountered with the transports 41and 42.

To more fully understand the invention, reference will now be made to aspecific embodiment of a transport, such as transport 42, and a specificembodiment of the position sensors, such as sensors 116x and ll6Y,associated therewith. In FIG. 2, a frame assembly 70 is adapted to holdthe diapositive 125 which is to be scanned by the scanner 26. The frameassembly 70 rests upon a flat base assembly, which includes a plate 71and is provided with a suitable surface to provide easy movement of theframe 70 across the plate 71. Bearing feet or pads have been used on theunderside of one frame assembly 70 and were found to work well. A pairof geared driving racks 72 and 73 each carry at one end thereof aframe-engaging end member 74, 75, respectively. Members 74 and 75 eachcarry a pair of rollers 76 which rotate about horizontal axes and rideon the upper surface of the plate 71. Second pairs of rollers 77,supported for rotation about vertical axes on each of the members 74 and75, engage the edges 102, 103 of the frame assembly 70. The outer endsof the racks 72, 73 are supported by rollers 78. The edges 102, 103 ofthe frame assembly 70 are flat surfaces which intersect at an angle offor the particular system illustrated herein. The racks 72 and 73 aremaintained perpendicular to the surfaces 102 and 103, and therefore theracks, if extended, would intersect at an angle of 90.

The frame 70 is maintained in engagement with each of the rollers 77 byany suitable assembly. In the embodiment illustrated, the oppositecorners 79, 80 of the frame 70 have the ends of cables 81 and 82 securedthereto. Cable 82 passes around a guide 83 supported on plate 71, andthen around guide 84 also supported on plate 71. ln a similar manner,the cable 81 passes around the guide 84. Two cables extend from guide 84around a guide 84 supported for rotation about a horizontal supportlocated in the plane of the plate 71, through an opening in the plate 71adjacent guide 85, and around a pulley 86 having a weight 87 securedthereto. Through pulley 86, the cables extend through an opening, notillustrated, in plate 71, around an adjustment eccentric 88, and aresecured by their respective ends to clamps 89 attached to the plate 71.The arrangement is such that the weight 87 acting through the cables 81and 82 provides a yielding force to the frame 70, which arches the edges102 and 103 into engagement with the rollers 77 associated with driveracks 72 and 73. Rotation of the eccentric 88 is used for initialadjustment of the cables.

A drive gear 91 secured to the drive shaft of the reversible electricstepping motor 22 engages the rack 72. in a similar manner, a drive gear110 secured to the drive shaft of the second reversible electricstepping motor 20 engages the drive rack 73. The operation of thereversible electric stepping motors 22, 20 is interrupted by operationof an associated control switch by arms 93, 111, having rollers 94 and112 engaged with the non-toothed edge of the associated racks 72 or 73.The arms 93 and 1 1 1 are pivoted on pins secured to the plate 71. Theswitches 97 and 113 associated with the arms 93 and 111 are adapted tobe operated whenever the respective arm is moved clockwise. The endmembers 74 and 75 on the racks 72 and 73 have beveled surfaces, such assurface 92 on member 74, which engage the rollers 94 and 112 wheneverthe racks reach their maximum extent of outward movement. Therefore, theassociated switch 97 or 113 will be actuated to interrupt further driveof the rack. Corresponding beveled surfaces, such as surface 92A on rack72, on the outer ends of racks 72 and 73 serve the same purpose when themaximum extent of inward travel has been reached.

An optical system, including a lens 101, is aligned with theintersection of the lines of travel of racks 72 and 73 so that thecenter of scanning corresponds to this intersection. More specifically,this intersection occurs at the intersection of the pitch lines of thedrive racks 72 and 73.

The diapositive 128 can be held in position in the frame 70 by variousmeans. For purposes of illustration, the diapositive 12B is secured to abacking plate 70A. A corner stay 98 is shown as being in engagement withone corner of the plate 70A with springs 99, 100 urging the corner stay98 and plate 70A toward the opposite corner of the frame 70.

Though a photo-positioning device, such as illustrated in FIG. 2, iscapable of very accurate and precise positioning of the diapositive 128,it is apparent that position errors may yet arrive from several sources.First, the fact that the motors 22 and 20 operate in an incrementalstepping mode places a basic limitation on position resolution as beingdetermined by the smallest increment of movement provided by motors 22,20 as coupled through the drive gears 91, 110. Second, wear of the gears91, and the corresponding toothed portions of racks 72 and 73 provides avariable position error with time. In addition, the manufacturingtolerances of these elements and the necessity for some play so as toavoid binding introduces other position errors.

In order to compensate for position errors, the position sensors 116Y,116X include, respectively, incremental rotary encoders 130, 140, whichare mounted on supporting brackets 131, 141 attached to a portion of thesupport plate 71. The encoders 130, have shafts 133, 142 which extendthrough and are supported by brackets 131, 141. Pulleys 132, 143 areattached to shafts 132, 142 and rotate therewith. Cables 134, 144attached at one end to the ends 135, 145 of racks 72, 73, respectively,pass over pulleys 133, 143, pass back over pulleys 136, 146 rotatablyjournalled in mounting brackets 131, 141, and are attached at theirother end to weights 137, 147. The function of pulleys 136, 146 andweights 137, 147 is to provide a constant tension on the cables 134,144, so as to prevent any slack therein. As a result, the pulleys 133,143 rotate by an amount directly proportional to the translativemovement of racks 72, 73 in their respective Y and X directions.

The encoders 130, 140 can be of a type which provides an output signalrepresenting the actual displacement of the encoder shaft from areference position, better known as an absolute encoder, or of a typewhich provides an output signal proportional to each predeterminedincrement of displacement of the encoder's shaft, better known as anincremental encoder. [n a preferred embodiment, the encoders 130, 140comprise an incremental encoder, each operative to provide two digitalpulse train outputs on lines 130', 130" and 140', 140", respectively.Each of the pulse train outputs changes its logic state for everypredetermined increment of rotation of the associated shaft 132 or 142.In addition, the two pulse trains provided by each encoder are displacedin phase by 90. The outputs from encoder 130 are seen in FIG. 3.

Encoders of this type are commercially available from SequentialInformation Systems, lnc., Elmsford, N.Y., as their model SOOOIDzPAl.

The pulse trains on lines 130, 130" and 140', 140" are supplied tosynchronous phase detectors 150, 154, respectively, and the signals onlines 130', 140' are additionally supplied to the input of reversiblecounters 152, 156. The direction of counting of counters 152, 156 iscontrolled by output signals from the phase detectors 150, 154 presenton lines 151A, 1518, and A, 1558. The outputs of the reversible counters152, 156 serve as the position sensor outputs 122Y, 122K, respectively.

The phase detectors 150, 154 may be of any type well known to the artfor providing binary output signals whose logic state represents thedynamic phase relationship between the input pulse trains thereto. Thatis, the logic state of the outputs on lines 151 and 155 represents whichof the two input pulse trains is leading in phase at a particular pointin time for a given direction of movement of racks 72 and 73.

In the embodiment shown in FIG. 2, the phase detectors each comprise aJ-K flip-flop with the pulse train on lines 130', 140' being applied tothe J steering inputs, and the pulse train outputs on lines 130", 140"being applied to the clock [C] inputs. The K steering inputs aregrounded. The Q outputs are connected to lines 155A, and the Q outputsare connected to lines 1518, 155B. The Q and Q outputs on lines 151A,155A are seen in FlG. 3.

The operation of the phase detectors can be understood from aconsideration of FIG. 3 taken in conjunction with the specificconnections in FIG. 2 for encoder 130 and phase detector 150.

At the reference positions y the Q and Q outputs of the phase detector150 are logic and logic I, respectively. If the rack 72 now is moved ina first direction D indicated by the arrow in FIG. 3, the signal on line130", or the clock signal, goes from logic I to logic 0 at distance y Atthis transition, the signal on lines 130', or the position signal, is alogic I. Therefore, the flip-flop within phase detector 150 switches sothat the 0 output is a logic 1 and the Q output is a logic 0.

If the rack 72 now is moved in an opposite direction D the clock signalgoes from logic 0 to logic I. and then from logic I to logic 0 atdistance y,. At this transition, the position signal is a logic 0, andtherefore the flip-flop within phase detector 150 switches so that the 0output is a logic 0 and the Q output is a logic l.

[t can be recognized that at every logic I to logic 0 transition of theclock signal, the relative phase of the position signal is investigatedby phase detector 150. if the position signal is a logic 1 at thetransition, the rack 72 is displaced in the first direction, and ifalogic 0, the rack 72 is displaced in the second, opposite direction. Thefirst direction is denoted by a logic 1 signal on the 0 output, and thesecond direction is denoted by a logic 1 signal on the 0 output.

The reversible counters 152 and 156 are set at zero, or some otherestablished count, when the associated racks 72, 73 are positioned atthe reference position. For displacement of the racks 72 and 73 fromthose reference positions, counters I52 and 156 are incremented by thepulses on lines 130' and 140 in the direction controlled by the Q and Ooutputs from phase detectors 150 and 154. Accordingly, counters 152 and156 contain digital numbers whose magnitudes represent the displacementof the associated rack 72 or 73 from the reference position, and whosesigns represent the directions of displacement.

The signals on lines 122Y, 122x are in turn supplied to comparator 124,as heretofore described. in FIG. 2, comparator 124 is seen to comprisetwo summing junctions 124Y, 124X, for receiving the digital numberspresent on lines 122Y, 122X representing the actual positions of theassociated racks 72 and 73, and for receiving, on lines 121Y and 121X,the digital numbers supplied from computer 29 representing the desiredstage coordinate positions for racks 72 and 73. Summing junctions 124Yand 1204 can be any digital comparators, and preferably are withincomputer 29.

The signals on lines 124Y, 121Y, and 124x and l2lX are subtractivelycombined so that the output error signals, on lines 125Y, 125x comprisedigital numbers representing the difference between the desired andactual positions of the racks 72, 73. As previously described, thesesignals are supplied to raster shaper 63.

Now turning to FIG. 4, which is a block diagram of a preferredembodiment of the raster shaper 63 (which is identical to raster shaper62), it can be seen that the X and Y deflection waveforms for a squareraster delivered from the scan generator 56 on lines 170 and 171,respectively, are modified by multiplier circuits 172 and 173,respectively, to provide deflection signals at different amplitudes onlines 174 and 175. Such signals appear on output lines 176 and 177 afterpassing through the summing networks 178 and 179, respectively.

it will also be seen that the X and Y waveforms will be modified furtherby the addition at summing circuits 178 and 179, respectively, of othersignals as described below. In particular, the X output signal will bethe resultant of the signal on line 174, already described, and a Ydeflection signal delivered to summing point 178 on line 180 frommultiplier 181 on line 182. Similarly, the Y output signal will be theresultant of the signal on line 175, already described, and the Xdeflection signal delivered to summing point 179 on line 183 frommultiplier 184. The multiplier units can conveniently be conventionaldigital-to-analog converters with a variably controlled reference beingthe analog input. Thus, they are labeled as D/A."

As a result of the action of the elements of the raster shaper so fardescribed, the scale and shape of a raster will be altered in responseto computer signals K1, K2, K4 and K5, in FIG. 3, to compensate for thefirst order effects of irregularities in the flight line of the surveyaircraft and orientation of the photographic cameras at the moment ofexposure.

Referring again to FIG. 3, it will be seen that the AZ signal from thecorrelator 49 represents variation of the terrain height in the modelarea being scanned. The multipliers 186 and 187 distribute the AZ signalto the X and Y axes of the camera so as to accommodate transformation inaccordance with the photogrammetric of geometric aspects of the model atthe moment of exposure in the aircraft.

The computer 29 provides the raster-shaping coefficients Kl-K6, on lines66, 67 of FIG. 1, in addition to the center-point coordinates for thetransports 41, 42 on lines 44X, 44Y, 45X, 45Y, in accordance withtechniques which per se are known in the art.

In addition, the X and Y waveforms are modified further by the signalsfrom the comparator 124. Specifically, the X error signal present online 125x is first converted into analog form by a digital-to-analogconverter 190 and then applied to summing junction 178, and the Y errorsignal on line 125Y is converted into analog form by a digital-to-analogconverter 19! and applied to summingjunction 179. As a result, the X andY waveforms are modified so that the raster produced on scanner 26 isshifted in both X and Y in a direction controlled by the sign of the Xand Y error signals and in an amount proportional to the magnitudethereof.

It will thus be apparent that a highly accurate and precise automaticorthophoto display and printing system has been described, which mayfind additional utility in the orthomapping of contour information, aswell as improved resolution of X and Y parallax, due to the accurate andprecise positioning of the diapositives 12A and 128.

Though the invention has been described with respect to a preferredembodiment thereof, it is to be understood by those skilled in the artthat the invention is not limited thereto. For example, the positionsensors, such as sensor 116X, 116Y can comprise linear absolute orincremental encoders requiring no mechanical connection to the racks 72,73. In one embodiment, optical sensing equipment can be used comprisingan optical sensor disposed adjacent each of the racks 72, 73, with adual track of a plurality of finely divided black and white coding marksbeing placed on racks 72 and 73. [n such a case, signals derived fromthe transition from one track would be used to increment a reversiblecounter, as previously described, and the transitions from the otherwould control the direction of counting of the counter. Otherincremental and absolute encoders and position sensors will be readilyapparent to those skilled in the art. Therefore, the limits of thepresent invention are intended to be bounded only by the appendedclaims.

I claim:

1. In an automatic orthophoto display system including first and secondscanning means, each of said scanning means establishing a raster, saidfirst and said second scanning means providing video output signalsrepresenting the information in a common area of first and secondphotographs making up a stereo pair, respectively; means providingcoordinate signals for establishing desired positions for each of saidfirst and second photographs relative to said first and said secondscanning means, respectively; first and second photopositioning meansresponsive to said coordinate signals for moving said first and secondphotographs relative to said first and second scanning means,respectively; means responsive to said video output signals formodifying said coordinate signals for at least one of saidphoto-positioning means to cause the associated one of said photographsto be moved in a direction relative to the associated scanning means toreduce parallax error at one point in the common area of saidphotographs scanned by said raster; signal correlating means responsiveto said video output signals to derive an error signal proportional totiming differences between homologous components of the video outputsignals throughout the common area of said photographs scanned by saidrasters; and raster-shaping means coupled to said signal correlatingmeans and to said first and to said second scanning means and responsiveto said error signal to distort the scan pattern of at least one of saidscanning means, said distortion being in a direction to reduce parallax:the improvement comprising means sensing the actual position of at leastone of said photopositioning means relative to its associated scanningmeans, means comparing the actual position so sensed with the desiredposition of said photo-positioning means to derive a position errorsignal proportional to the difference therebetween, and means couplingsaid comparator to said raster-shaping means so that said raster-shapingmeans shifts the raster of said associated scanning means in response tosaid position error signal.

2. The improvement for an automatic orthophoto display system as recitedin claim 1, wherein:

a. said sensing means senses the actual position of said first and saidsecond photo-positioning means;

b. said comparing means derives first and second position error signalsfor said first and said second photo-positioning means; and,

c. said raster-shaping means shifts the raster of each of said first andsaid second scanning means in response to said first and said secondposition error signals, respectively.

3. The improvement for an automatic orthophoto display system as recitedin claim 1, wherein:

a. said sensing means senses the actual position of said onephoto-positioning means in orthogonal coordinate directions;

b. said comparing means includes means for deriving position errorsignals for each of said coordinate directions; and,

c. said raster-shaping means includes means operative to shift saidraster in either of said coordinate directions in response to saidposition error signals.

4. The improvement for an automatic orthophoto display system as recitedin claim 1, wherein said sensing means includes an encoder means forproviding an output pulse for every predetermined increment of movementof said one photo positioning means along a given coordinate axis, adirection detection means providing an output direction signalrepresenting the direction of said movement along said coordinate axis,and bidirectional, reversible counter means operative to accumulate saidoutput pulses in a given direction in response to said output directionsignal to provide a digital signal representing said actual position.

5. An automatic orthophoto display system comprising:

a. a photo scanning system including first and second scanning meansrelatively aligned with first and second photographs making up a stereopair, and a scan generator providing deflection output signals to saidfirst and said second scanning means to establish a scanning raster foreach of said scanning means, each of said first and said second scanningmeans thereby providing a video output signal representing theinformation on the portion of the corresponding photograph scanned bysaid raster;

b. means providing a plurality of first signals establishing desiredpositions of said first and said second photographs relative to saidfirst and said second scanning means;

c. means positioning said first and said second photographs in responseto said plurality of first signals;

d. parallax correction means responsive to said video output signalsincluding means for developing therefrom a parallax correction signal,and means summing said parallax correction signal with those of saiddeflection output signals which are supplied to at least one of saidfirst or second scanning means to thereby perturb the scan pattern ofsaid one of said first and second scanning means in a direction so as toreduce parallax between said first and said second photographs;

e. sensor means providing a second signal propor tional to the actualposition of at least one of said photographs relative to the associatedone of said scanning means;

f. comparator means providing an output signal proportional to thedifference between said first signal establishing the desired positionof said one of said photographs and said second signal; and,

g. means summing said output signal with those of said deflection outputsignals which are supplied to said scanning means associated with saidone of said photographs.

6. An apparatus for providing accurate and precise scanning of anobject, comprising:

a. scanning means including means establishing a scan pattern;

b. frame means relatively aligned with said scanning means for holdingthe object;

0. means providing a first signal establishing a desired position ofsaid object relative to the scan pattern of said scanning means;

d. drive means coupled to said frame means for moving said object inresponse to said first signal,

e. sensor means providing a second signal proportional to the actualposition of said object relative to said scan pattern;

f. comparator means for subtractively combining said first and saidsecond signals to obtain a position error signal; and,

g. means coupled to said comparator and to said scanning means forshifting the scan pattern of said scanning means in proportion to saidposition error signal.

7. An apparatus as recited in claim 6, wherein said sensor meansincludes:

a. an incremental encoder having an input member;

b. means coupling said input member to said frame means;

c. said incremental encoder including means providing first and secondoutput pulse trains in response to movement of said input member, saidpulse trains being displaced in phase by substantially 90", a pulse ineach of the pulse trains being provided for every predeterminedincrement of movement of said input member;

d. bidirectional, reversible counter means having an input terminal, anoutput terminal on which appears a signal representing the accumulatedpulses presented to said input terminal, and a control terminalcontrolling the direction of counting thereof;

e. means coupling one of said first and second output pulse trains tosaid input terminal;

f. synchronous phase detection means having both said first and saidsecond output pulse trains coupled thereto and operative in response tothe relative phase of said first and second output pulse trains toprovide a direction control signal representing the direction ofmovement of said input member, and therefore of said frame member, alongsaid axis; and,

g. means coupling said direction control signal to said control terminalof said counter.

8. An apparatus as recited in claim 7, wherein said incremental encodercomprises a rotary incremental encoder in which said input membercomprises a rotatable shaft, and wherein said means coupling said inputmember comprises a pulley rigidly rotatable with said shaft, a cableattached at one end to said frame member, and passing over said pulley,and tensioning means attached to the other end of said cable.

9. An apparatus as recited in claim 8, wherein said tensioning meanscomprises a second pulley spaced apart from said first pulley andsupported for independent rotation, said cable passing around saidsecond pulley, and a weight member suspended from the portion of saidcable which is passed around said first and said second pulleys.

10. An apparatus as recited in claim 7, wherein said synchronous phasedetection means comprises a J-K flip-flop.

1. In an automatic orthophoto display system including first and secondscanning means, each of said scanning means establishing a raster, saidfirst and said second scanning means providing video output signalsrepresenting the information in a common area of first and secondphotographs making up a stereo pair, respectively; means providingcoordinate signals for establishing desired positions for each of saidfirst and second photographs relative to said first and said secondscanning means, respectively; first and second photo-positioning meansresponsive to said coordinate signals for moving said first and secondphotographs relative to said first and second scanning means,respectively; means responsive to said video output signals formodifying said coordinate signals for at least one of saidphoto-positioning means to cause the associated one of said photographsto be moved in a direction relative to the associated scanning means toreduce parallax error at one point in the common area of saidphotographs scanned by said raster; signal correlating means responsiveto said video output signals to derive an error signal proportional totiming differences between homologous components of the video outputsignals throughout the common area of said photographs scanned by saidrasters; and raster-shaping means coupled to said signal correlatingmeans and to said first and to said second scanning means and responsiveto said error signal to distort the scan pattern of at least one of saidscanning means, said distortion being in a direction to reduce parallax:the improvement comprising means sensing the actual position of at leastone of said photo-positioning means relative to its associated scanningmeans, means comparing the actual position so sensed with the desiredposition of said photo-positioning means to derive a position errorsignal proportional to the difference therebetween, and means couplingsaid comparator to said raster-shaping means so that said raster-shapingmeans shifts the raster of said associated scanning means in response tosaid position error signal.
 2. The improvement for an automaticorthophoto display system as recited in claim 1, wherein: a. saidsensing means senses the actual position of said first and said secondphoto-positioning means; b. said comparing means derives first andsecond position error signals for said first and said secondphoto-positioning means; and, c. said raster-shaping means shifts theraster of each of said first and said second scanning means in responseto said first and said second position error signals, respectively. 3.The improvement for an automatic orthophoto display system as recited inclaim 1, wherein: a. said sensing means senses the actual position ofsaid one photo-positioning means in orthogonal coordinate directions; b.said comparing means includes means for deriving position error signalsfor each of said coordinate directions; and, c. said raster-shapingmeans includes means operative to shift said raster in eiTher of saidcoordinate directions in response to said position error signals.
 4. Theimprovement for an automatic orthophoto display system as recited inclaim 1, wherein said sensing means includes an encoder means forproviding an output pulse for every predetermined increment of movementof said one photo positioning means along a given coordinate axis, adirection detection means providing an output direction signalrepresenting the direction of said movement along said coordinate axis,and bidirectional, reversible counter means operative to accumulate saidoutput pulses in a given direction in response to said output directionsignal to provide a digital signal representing said actual position. 5.An automatic orthophoto display system comprising: a. a photo scanningsystem including first and second scanning means relatively aligned withfirst and second photographs making up a stereo pair, and a scangenerator providing deflection output signals to said first and saidsecond scanning means to establish a scanning raster for each of saidscanning means, each of said first and said second scanning meansthereby providing a video output signal representing the information onthe portion of the corresponding photograph scanned by said raster; b.means providing a plurality of first signals establishing desiredpositions of said first and said second photographs relative to saidfirst and said second scanning means; c. means positioning said firstand said second photographs in response to said plurality of firstsignals; d. parallax correction means responsive to said video outputsignals including means for developing therefrom a parallax correctionsignal, and means summing said parallax correction signal with those ofsaid deflection output signals which are supplied to at least one ofsaid first or second scanning means to thereby perturb the scan patternof said one of said first and second scanning means in a direction so asto reduce parallax between said first and said second photographs; e.sensor means providing a second signal proportional to the actualposition of at least one of said photographs relative to the associatedone of said scanning means; f. comparator means providing an outputsignal proportional to the difference between said first signalestablishing the desired position of said one of said photographs andsaid second signal; and, g. means summing said output signal with thoseof said deflection output signals which are supplied to said scanningmeans associated with said one of said photographs.
 6. An apparatus forproviding accurate and precise scanning of an object, comprising: a.scanning means including means establishing a scan pattern; b. framemeans relatively aligned with said scanning means for holding theobject; c. means providing a first signal establishing a desiredposition of said object relative to the scan pattern of said scanningmeans; d. drive means coupled to said frame means for moving said objectin response to said first signal; e. sensor means providing a secondsignal proportional to the actual position of said object relative tosaid scan pattern; f. comparator means for subtractively combining saidfirst and said second signals to obtain a position error signal; and, g.means coupled to said comparator and to said scanning means for shiftingthe scan pattern of said scanning means in proportion to said positionerror signal.
 7. An apparatus as recited in claim 6, wherein said sensormeans includes: a. an incremental encoder having an input member; b.means coupling said input member to said frame means; c. saidincremental encoder including means providing first and second outputpulse trains in response to movement of said input member, said pulsetrains being displaced in phase by substantially 90*, a pulse in each ofthe pulse trains being provided for every predetermined increment ofmovement of said input member; d. bidirectional, reversible countermeans having an input terminal, an output terminal on which appears asignal representing the accumulated pulses presented to said inputterminal, and a control terminal controlling the direction of countingthereof; e. means coupling one of said first and second output pulsetrains to said input terminal; f. synchronous phase detection meanshaving both said first and said second output pulse trains coupledthereto and operative in response to the relative phase of said firstand second output pulse trains to provide a direction control signalrepresenting the direction of movement of said input member, andtherefore of said frame member, along said axis; and, g. means couplingsaid direction control signal to said control terminal of said counter.8. An apparatus as recited in claim 7, wherein said incremental encodercomprises a rotary incremental encoder in which said input membercomprises a rotatable shaft, and wherein said means coupling said inputmember comprises a pulley rigidly rotatable with said shaft, a cableattached at one end to said frame member, and passing over said pulley,and tensioning means attached to the other end of said cable.
 9. Anapparatus as recited in claim 8, wherein said tensioning means comprisesa second pulley spaced apart from said first pulley and supported forindependent rotation, said cable passing around said second pulley, anda weight member suspended from the portion of said cable which is passedaround said first and said second pulleys.
 10. An apparatus as recitedin claim 7, wherein said synchronous phase detection means comprises aJ-K flip-flop.